Wednesday, August 25, 2010

PIlot Charts

Pilot charts are very interesting and useful tools for planning a long passage. Below is a piece of the one for June around Bermuda. You can get a whole book of these (they're big) and there's a page for each month. The chart represents averages of data taken over many, many seasons.

The blue circles are the most interesting. Each one show for that location for that month the average wind direction and strength. For example, the one above with the 5 in the middle: The longest arrow shows wind from the south east. The length of the arrow shaft is proportional to the percentage of time the wind blows from that direction. The number of feathers on the arrows show the wind speed on the Beaufort scale. So for the one above the most persistant wind for that location for June is SE force 3. The 5 in the middle indicates 5% of the time calms.

The green lines indicate ocean currents and the direction and velocity in knots is given. When you look at the bigger picture from Antigua to Bermuda for June you see a general picture of fair winds for a northerly passage.

You'll note that if the percentage for a particular wind direction gets too high (above 29%) they don't make the arrow longer but just put the percentage withing the shaft.

Anyway, looks like E and SE winds most of the way and then switching to S - SW as we approach Bermuda. In between there are surely going to be some doldrums. Take more fuel! Another thing to notice is that when the wind switches to the north there's plenty of it. Those probably represent norther's coming down.

There's lots more info on the charts regarding temperatures, wave heights and general description of the persistent weather for that month. You can buy these charts at the usual chandleries or download them! Here's the link.

I'm visiting the captain of the Bounty today as the Tall Ships are in Chicago. Apparently my book, The Why Book of Sailing has been a popular volume on board. Who knew! A full report and pictures tomorrow.

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